Adding flavor to the result is always fun, this is why I always do something based on the roll when it seems exceptionnal. However, this doesn't change the fact that my players can call Raises. It's even better if they call Raises because it will go in the something they want, otherwise, it's stuffs that I wanna add up.
Oh and when I say "when it seems exceptionnal" it's "exceptionnally good or exceptionnally bad". If they have a Roll TN of 25 to do and someone rolls below 10, yes, that player will receive a something bad. It may be that he tripped while he attempts to hit his opponent causing him to fall on the floor and losing his weapon. It may be that he investigates a flower while looking at a crime scene because he thinks that the culprid hid the weapon inside the pot. And so more.
This adds up to the game, but you have to add this on both side, good or bad. This is even better when the players plays with the results, even if his character's sure that to get to the enemy headquarter he'll need to follow the Golden Brick Road and gets mad at the other characters because he's being ignored.